Incompatibilities / Differences between C and C++

Transcription

1 Incompatibilities / Differences between C and C++ 1. Objectives 1. The resolution operator 2. The operators new and delete 3. Inline functions 4. Functions returning a reference data type 5. Call by reference and call by value 6. The Boolean data type 7. Default parameters for functions 8. The reference type 9. Overloading functions 2. Incompatibilities between C and C Declaring and defining functions in C++ Alternative 1 Alternative 2 #include <iostream> using namespace std; void twotimes (int a) cout<<a; a=a*2; cout<<" doubled is "<<a; //system("color fc"); twotimes(10); _getch(); #include <iostream> using namespace std; void twotimes(int); twotimes(10); _getch(); void twotimes(int a) cout<<a; a=a*2; cout<<" doubled is "<<a; Both alternatives are correct in C++. Do not forget to declare the prototype of a function before call it. 2.2 Void pointers There is NO default conversion of pointers from the type "void*" in other types of pointers.

2 For the type (void*) C++ allows the default conversion from: the type of the pointer -> void* void * vp; int *ip; vp = ip; //correct in C and in C++ ip=vp; //correct in C, incorrect in C++ ip = (int*)vp; //correct in C and C++ 3. Improvements of C++ Suggestions: 1) Use more explicit names for functions and for variables. int countspaces(char* str) int counter; // some computations return counter; Is more explicit and used than: int cnt(char* str) int i; return i; 2) Try to add more comments when writing source code! 3) Do not put more than 90 characters on one line! 3.1 The boolean type The boolean type is a basic data type in C++, C does not have this data type. a program which returns true if an element is found in an array and false otherwise. bool find_elem() bool found=false; int i=0,n,elem, v[100]; cout<<"introduce the length of the array "<<endl; cin>>n; while (i<n) cout<<"v["<<i<<"]= "; cin>>v[i]; i++;

3 cout<<"introduce the element you search for "; cin>>elem; for (i=0;i<n;i++) if (elem==v[i]) found=true; return found; 3.2 I/O console (cin, cout) - We use cout for printing on the screen (cout console output stdout) - We use cin for storing a new entry (cin console input stdin) Both are included in the iostream library. In order to use them we have to include the iostream library and we have to declare the use of the standard namespace std using namespace std. 3.3 Facilities to statements In C++ you can declare the local variables anywhere inside the body of a function, but be careful to declare them before using them! #include <iostream> using namespace std; int sum (int a, int b) return (a+b); int x,y,z; cout<<"introduce x and y "<<endl; cin>>x>>y; //read x and y cout<<"the sum of "<<x <<" and "<<y<<" is "<<sum(x,y); //call the function directly in cout //only in the case when the function returns a value // (is not void) z=23+sum(10,23); cout<<"z= "<<z; char name[20]; cout<<"what is your name? "<<endl; cin>>nume; cout<<"wow! "<<nume<<" is such a beautiful name!";

4 3.4 Reference variables In C++ we can declare identifiers as references to objects of a certain data type. This reference variable has to be instantiated when declaring it with the address of a variable which is already defined. int number=10; int &refint=number; //reference to int, HAS TO be instantiated cout<<"the number is "<<number<<" and refint = "<<refint<<endl; refint=200; //automatically number=200; cout<<"after.. the number = "<<numar<<" and refint = "<<refint<<endl; An example of interchanging two numbers The C version void intersch(int *a, int *b) int aux; aux=*a; *a=*b; *b=aux; int a,b; cout<<"give a and b "<<endl; cin>>a>>b; intersch(&a,&b); cout<<"the interchanged values are "<<a <<" and "<<b; The C++ version void intersch1(int &a, int&b); int a,b; cout<<"give a and b "<<endl; cin>>a>>b; intersch1(a,b); cout<<"the interchanged values are "<<a <<" and "<<b; void intersch1(int &a, int&b) int auxiliary; auxiliary = a; a = b; b = auxiliary; If, for example, we have: int variabile; float &refvar=variabile; they have different types. The compiler will not create a reference to a variable int, but will allocate memory for a float variabile, therefore a hidden object will be created for the given reference. ATTENTION!! The data type of the reference variable and of the variable with which is instantiated has to be the same.

5 3 ways of parameter passing: void function_call_by_value(int x, int y, int z) x=10; y=20; z=30; void function_call_by_pointer_reference(int *x, int *y, int *z) *x=10; *y=20; *z=30; void function_call_by_reference(int &x, int &y, int &z) x=10; y=20; z=30; int x=10, y=20, z=30; int &refx=x; int &refy=y; int &refz=z; function_call_by_value(x,y,z); cout<<"by value "<<x<<" "<<y<<" "<<z<<endl; function_call_by_pointer_reference(&x,&y,&z); cout<<"by pointer "<<x<<" "<<y<<" "<<z<<endl; function_call_by_reference(refx,refy,refz); cout<<"by reference "<<x<<" "<<y<<" "<<z<<endl; 3.5 Functions which return references struct book char author[64]; char title[64]; float price; ; book library[3] = "Jamsa and Klander", "All about C and C++", 49.9, "Klander", "Hacker Proof", 54.9, "Jamsa and Klander", "1001 Visual Basic Programmer's Tips", 54.9; book& give_abook(int i) if ((i >= 0) && (i < 3)) return(library[i]); else return(library[0]);

6 void main(void) cout << "Almost getting the book \n"; book& a_book = give_abook(2); cout << a_book.author << ' ' << a_book.title; cout << ' ' << a_book.price; 3.6 Parameters with default values In C++ we can declare functions with parameters which have default values. This allows us to call the function in several ways. The arguments with default values have to be at the end of the parameter list. int divide(int a, int b=4) int result; result=a/b; return result; void function1(int, float=23.9, long=100); int main() cout<< divide(15)<<endl; cout<< divide(20,10)<<endl; // cout<< divide(); //incorrect float f=4.5; function1(11); // correct call function1(11,f);// correct call function1(11,2.6); //correct call // functie(11,100); // incorrect call return 0; void function1(int i, float f, long l) cout<<"i= "<<i<<" f= "<<f<<" l= "<<l<<endl; 3.7 Overloading function names C++ allows us to have two or more functions with the same name, but with a different number of parameters and a different return value.

7 #include <iostream> using namespace std; int operation(int a, int b) return (a*b); float operation(float a, float b) return (a/b); char * operation(char *s1, char *s2) char *result = new char[strlen(s1)+strlen(s2)+1]; strcpy(result, s1); strcat(result, s2); return result; int main () int x=5,y=2; float n=5.0,m=2.0; cout << operation(x,y); cout << "\n"; cout << operation(n,m); cout << "\n"; char *s1="the first string ", *s2="the second string "; cout<<"adding to strings "<<operation(s1,s2)<<endl; return 0; 3.8 Inline functions Syntax: inline data type name (parameters... ) instructions... You can use the inline functions when you have a small number of parameters and not many instructions. The advantage of using inline functions is an increased execution speed. The inline functions replace the macros built using #define and avoid the difficulties caused by these macros: - these functions do not return values - the parameters should be specified in parentheses, for an assessment with respect to the precedence of the operators.

8 #define Max(a,b) ((a)<(b))? (b) : (a) inline int Max(int a, int b) return (a<b)? b : a ; Study the following example which prints the running time when calling times each function #include <iostream> #include <time.h> using namespace std; inline void swap_inline(int *a, int *b, int *c, int *d) int temp; temp = *a; *a = *b; *b = temp; temp = *c; *c = *d; *d = temp; void swap_call(int *a, int *b, int *c, int *d) int temp; temp = *a; *a = *b; *b = temp; temp = *c; *c = *d; *d = temp; void main(void) clock_t start, stop; long int i; int a = 1, b = 2, c = 3, d = 4; start = clock(); for (i = 0; i < L; i++) swap_inline(&a, &b, &c, &d); stop = clock();

9 cout << "Time for inline: " << stop - start; start = clock(); for (i = 0; i < L; i++) swap_call(&a, &b, &c, &d); stop = clock(); cout << "\nthe running time for calling the function is: " << stop - start; 3.9 New operators: new and delete In C the memory management is implemented in auxiliary libraries and the user can use some functions. Until now, in all our programs, we have only had as much memory available as we declared for our variables, having the size of all of them to be determined in the source code, before the execution of the program. But, what if we need a variable amount of memory that can only be determined during runtime? For example, in the case that we need some user input to determine the necessary amount of memory space. In C++ we have two new operators for memory management (dynamic memory): - new for memory allocation - delete the memory is freed int main() char * str = new char [100]; delete [] str; What is the effect of the following program? void main(void) char *array = new char[256]; int i; for (i = 0; i < 256; i++) array[i] = 'A'; for (i = 0; i < 256; i++) cout << array[i] << ' ';

10 3.10 The resolution operator #include <iostream> using namespace std; int global_variable=300; int global_variable=100; cout<<"the value of the local variable is "<< global_variable<<endl; cout<<" the value of the global variable is "<<::global_variable<<endl; Keywords in C++ asm auto bad_cast bad_typeid bool break case catch char class const const_cast continue default delete do double dynamic_cast else enum except explicit extern false finally float for friend goto if namespace new operator private protected public register reinterpret_cast return short signed sizeof static static_cast struct switch template this throw true try type_info typedef typeid typename union unsigned using virtual void

11 inline int long mutable volatile while xalloc Remark: The keywords cannot be used as names for variabiles, functions, classes etc. Operators in C++ The C++ operators (from the highest to the lowest priority) are: :: Scope resolution None :: Global None [ ] Array subscript Left to right ( ) Function call Left to right ( ) Conversion None. Member selection (object) Left to right -> Member selection (pointer) Left to right ++ Postfix increment None -- Postfix decrement None new Allocate object None delete Deallocate object None delete[ ] Deallocate object None ++ Prefix increment None -- Prefix decrement None * Dereference None & Address-of None + Unary plus None - Arithmetic negation (unary) None! Logical NOT None ~ Bitwise complement None sizeof Size of object None sizeof ( ) Size of type None typeid( ) type name None (type) Type cast (conversion) Right to left const_cast Type cast (conversion) None dynamic_cast Type cast (conversion) None reinterpret_cast Type cast (conversion) None static_cast Type cast (conversion) None.* Apply pointer to class member (objects) Left to right ->* Dereference pointer to class member Left to right * Multiplication Left to right / Division Left to right % Remainder (modulus) Left to right + Addition Left to right - Subtraction Left to right << Left shift Left to right >> Right shift Left to right < Less than Left to right > Greater than Left to right <= Less than or equal to Left to right

12 >= Greater than or equal to Left to right == Equality Left to right!= Inequality Left to right & Bitwise AND Left to right ^ Bitwise exclusive OR Left to right Bitwise OR Left to right && Logical AND Left to right Logical OR Left to right e1?e2:e3 Conditional Right to left = Assignment Right to left *= Multiplication assignment Right to left /= Division assignment Right to left %= Modulus assignment Right to left += Addition assignment Right to left -= Subtraction assignment Right to left <<= Left-shift assignment Right to left >>= Right-shift assignment Right to left &= Bitwise AND assignment Right to left = Bitwise inclusive OR assignment Right to left ^= Bitwise exclusive OR assignment Right to left, Comma Left to right Remarks: a) Operators can be oveloaded. b) except for the following ones:.,.*, ::,? :, #, ##. c) New operators introduced: new, delete,.*, ->,*. 4. Exercises 4.1 Print on the screen a text from a txt file test.txt (in C version) int main() //system("color fc"); char ch; FILE* fp=fopen("test.txt", "r"); if(fp==null) cout<<"error when trying to open the text file" <<endl; return 0; while((ch=fgetc(fp))!=eof) cout<<ch; fclose(fp); _getch(); return 0; 4.2 Search how many times a word (introduced by the user) occurs in a text file. 4.3 Write a program which decreasingly sorts a set of words. The user introduces the words. For each operation define a separate function.